The Zoologist — November, 1873. 3745 



is only applicable to those animals and plants naturally living between tide- 

 marks ; for, except in the case of some of the blennies, and a few other 

 small fishes, this exposure to the atmosphere is unsuited to the habits of 

 free-swimming animals. The appearance of the Actinise, and other soft- 

 bodied creatures, when left thus exposed, is often so unlike their condition, 

 when immersed, as to give rise to the suspicion of their being in an 

 unnatural state; but experience tells us that these animals thrive best 

 when subjected to this periodical exposure, and they show renewed vigour 

 on the subsequent rising of the artificial tide. A great advantage is also 

 gained by returning the water in a small stream to its place in the tank ; 

 by this means the whole of it undergoes purification in its passage through 

 the air, and becomes better fitted to support life. We need only take a 

 glance at the rocks at low water to satisfy ourselves that periodical exposure 

 to the atmosphere is almost one of the necessary conditions of life with 

 many species of marine animals. Some of the sea anemones are found so 

 high up in the I'ange of tide that they must necessarily be out of the water 

 for at least six hours of the twelve occupied by its ebb and flow ; and if we 

 go lower down and look under the large boulders, and into the dark 

 crevices of the rocks, we shall find a multitude of creatures, and many of 

 them of the most delicate structure, which, for an hour or two in every tide, 

 are quite out of reach of the water. Marine animals can bear this exposure 

 better than the inhabitants of fresh water ; the salts held in solution in 

 sea-water retain a moist atmosphere around the bodies of the various 

 animals which have been bathed with it, and evaporation consequently does 

 not take place very rapidly, so that we need not fear imitating Nature even 

 in what, at first sight, appears to be a hazardous proceeding." — 'Handbook 

 to the Fish House,' p. 10. 



Here we have the principle of the salt-water aquarium clearly 

 explained ; and I have, as in one or two previous instances, 

 italicised a passage to which I wish particularly to invite attention. 

 Nothing can be more truthful than the entire extract, and it is diffi- 

 cult to conceive anything more neatly expressed than the sentence 

 I have printed in italics ; Mr. Holdsworth's method of returning 

 the water into the aquarium is perhaps somewhat superseded by 

 the more energetic operation of the steam-engine, but the idea is 

 the same, and is perfectly in accordance with present practice. 



I believe it was during the year following the successful opening 

 at Regent's Park, that the late Dr. Robert Ball introduced a novel 

 mode of aeration at Dublin : this is spoken of as "a method of 

 keeping the sea-water in occasional motion by passing bubbles of 

 air through it from a pair of bellows worked by visitors." 1 canuot 

 find at this moment any categorical description of the experiment. 



